Broad-band and high speed guided wave lithium niobate modulators are important components for emerging wideband communication and signal processing systems. Various types of high speed modulator-switches have been reported as, for example in: (1) "Waveguide Electrooptic Modulators" IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-30, No. 8, pp 1121-1137, Aug 1982; (2) "Performance and Modeling of Broadband LiNbO.sub.3 Traveling Wave Optical Intensity Modulators", Journal of Lightwave Technology, Vol. 12. No 10, pp 1807, October 1994; (3). "High Speed Shielded Velocity-Matched Ti:LiNbO.sub.3 Optical Modulator". IEEE Journal of Quantum Electronics. Vol. 29. No. 9, pp 2466, September 1993; and (4) "Modeling and Optimization of Traveling Wave LiNbO.sub.3 Interferometric Modulators", IEEE Journal of Quantum Electronics, Vol. 27, No. 3, pp 608, March 1991. Among them, the interferometric type modulator using Y-branches is the most widely used structure for broadband modulation because of its simplicity. As desired modulation and switching speed increase, limitations in available drive power and power dissipation demand that these modulators operate at the highest electrical efficiency.
In the typical modulator design, the input optical waveguide is split via a first Y-junction into two collinear waveguides and RF electrodes are positioned parallel to these guides. As the optical wave propagates, the phase difference between the two waveguides is modulated by a traveling RF wave. When the two beams are combined via a second Y-junction, the output is either a maximum or a minimum, depending on whether the cumulative phase difference is an even or odd multiple of .pi. radians, respectively. This is the fundamental operating principle of the optical modulator.
Several years back, it was recognized that one of the fundamental problems with achieving high modulation bandwidth with lithium niobate modulators was the mismatch between the optical and RF propagation velocities. This mismatch arises because the optical refractive index of lithium niobate is about 2.2 while the RF refractive index is about 6.3. Thus, the RF power travels much slower than the optical power. Therefore, as the two forms of power propagate down the waveguide, the RF wavefront falls behind the optical wavefront. This results in reduced modulation efficiency at the higher frequencies, and the modulation bandwidth becomes frequency limited.
However, the velocity mismatch problem has been largely solved. It was recognized that, although the high RF dielectric constant of lithium niobate (.congruent.40) results in the velocity mismatch problem, suitable design of the RF waveguide parameters, such as electrode width, gap and thickness, and suitable choice of buffer layer thickness (for example 1.2 .mu.m of SiO.sub.2), cause more of the RF energy to travel in the region outside the lithium niobate so that the effective dielectric constant is reduced to 5. The RF refractive index now drops close to 2.2, and velocity mismatch is no longer the limiting factor in bandwidth of the modulator. This optimized design for a velocity matched modulator, since it relies on redistributing the RF power between the substrate and the surrounding region, depends only on the waveguide parameters and not on the properties of the specific metal used for the electrode.
As frequency increases, the cost of drive electronics increases significantly for any given drive voltage. Therefore, at the higher frequencies, above 10 GHz, one would like to design modulators with very long electrodes so that the RF power needed for modulation would be minimized. In theory, with the velocity mismatch problem solved, increasing the electrode length should have no adverse effect. In practice, this is not the case. A new limiting factor--conductor loss in the electrode--enters the picture.
As is well known, , the penetration of the RF field into a metal conductor decreases as frequency increases. The extent of penetration is conventionally characterized by the skin depth. Thus, the volume within the conductor in which the RF power travels decreases with increasing frequency. It can be shown that this results in an ohmic loss in the conductor, which increases as the square root of the frequency, linearly with electrode length, and limits the bandwidth of the modulator. Conventionally, velocity matched electrodes have been fabricated using electroplated gold electrodes.
It is the primary object of this invention to show that the gain bandwidth product of velocity matched modulators can be increased by as much as 40% by the use of low resistivity metals such as copper instead of gold and by 47% by the use of silver instead of gold. Thus, for example, the highest bandwidth of a known commercially available modulator today is 40 GHz. This can be increased to about 56 GHz by use of copper. Conversely, the drive voltage of a known commercial 10 GHz modulator, used in the OC192 communications architecture, can be reduced from 7 volts to 3.5 volts by using copper instead of gold, without sacrificing bandwidth. This will reduce the cost of the drive electronics significantly.